Procedural polka dot pattern with cycles
OSL is nice but does not run on GPU which limits performance. The Polka pattern is not that hard to construct procedurally using nodes:

Start with a simple dot - a spherical gradient with Color Ramp:



With the ColorRamp you can control the size of the dot.

Next create tiling for procedural textures, but staggered. Extra is a Mapping node to control the size of the whole pattern:



That's it. You can also map it with Cylindrical or Spherical projection.